Boy - That is a can of worms you just opened. Get ready for information overload... LOL
The Colt stuff does have mating problems. They have different specs than a lot of the manufacturers, so if you don't like what's on the weapon, you are going to pay for the right aftermarket pieces to make them fit.
That said, a lot of the Colt stuff I have seen is good quality gear that seems to run just fine. I have never owned one, but I have seen a few come through the shop and no one that ever handed me one said it was a POS.
I did some checking and I can't find exactly what you are describing, but I did find a similar model that might be what you are talking about:
NAMEMatch Target Competition HBAR II
- Semi Auto
- Gas operated
- Multi-lug rotary locking bolt
RIFLING TWISTRH. 1 Turn in 9”
FRONT SIGHTAdjustable post for elevation
REAR SIGHTAdjustable for windage and elevation
SPECIAL FEATURES- Flat top receiver allows for scope mounting
This is a spec off the Colt home page for a model that has the 16" barrel and the fixed stock, with the flat top you are describing.
The chamber is cut for .223 only - which means you can't really shoot the 5.56 NATO stuff without doing some damage. The 1 in 9 twist barrel is for a bit heavier rounds. A lot of stock ammo for the AR category is .55gr but this is more along the lines of, I think, a .62 grain type of bullet.
There are some master reloaders here that can verify that for you, reloading is not my specialty by any means.
They seem to be retailing for around $1100 around the 'net with a casual glance. I would be a little leary at shelling out $850 for a rifle that is 5 to 6 years old without doing a bore scope and a close inspection. I mean, you are only $200 to $250 off of a brand new model if you REALLY want the Colt.
That said, there are much better deals out there on AR's that will perform as well, if not better, that won't have the aftermarket concerns of the Colt. They are a fine weapon, don't get me wrong, but you are paying for a name - and as long as you are okay with that, then more power to you.
There is a thread in the Knowledge Base that will show you how to build your own AR lower receiver with parts that you can purchase piece meal from places like Midwayusa and Brownell's, along with a bevy of specialized dealers, if you are interested. It is for registered members, so you would have to sign up to help support the site, but the option is there if you wish.
Best of luck -